Environmental considerations are driving increased use of environmentally-friendly chemicals in manufacturing. Environmental considerations include whether the chemicals are toxic, whether they have ozone-depleting properties, whether the chemicals are available from renewable resources, and whether they are biodegradable or recyclable. Chemical cleaning agents, used in a variety of situations including the manufacture of electrical and mechanical components to remove grease, solder flux, cutting oil and other contaminants from the manufacturing processes, are being examined in light of these environmental considerations. These environmental considerations, coupled with advances in solvent chemistries, have resulted in increased use of low-volatility cleaning agents for use with water rinses, commonly referred to as semiaqueous cleaners.
A primary advantage in the use of low-volatility cleaning agents is the reduction in amount of chemicals vaporized as part of the cleaning processes. However, because these cleaning agents do not evaporate rapidly, rinses are typically required to remove the cleaning agent from the substrate being cleaned.
Some of the semiaqueous cleaners in use involve limonene as a cleaning agent, followed by a water rinse to remove the cleaning agent. (Reference herein to limonene includes the isomers d- and l-limonene and any combination of these isomers.) Because limonene is not miscible in water, these cleaners require the addition of a surfactant to facilitate water rinsing. Limonene is specified in various U.S. patents to be used as a cleaning agent, typically to be blended with surfactants and water.
There are deficiencies in use of limonene as a cleaning agent, however. The surfactants used to allow water rinsing generally are nonvolatile materials, some of which remain on the substrate being cleaned in areas which tend to trap contaminants, such as in joints, in stranded wires, and in nonhermetically sealed assemblies. Further, these limonene and surfactant cleaning agents generally are not readily distilled because of the nonvolatile nature of the surfactants, complicating recycling of the cleaning agent for reuse. Finally, limonene is not fully effective in removing polar contaminants.
Additionally, alcohols are advantageous for cleaning because, typically, they effectively remove polar contaminants. Traditional short-chain linear alcohols such as isopropyl are flammable and, thus, are difficult to use in automatic cleaning equipment because of the fire and explosion hazards associated with flammable vapors around electrical equipment. The National Electrical Code imposes strict requirements upon equipment manufactured to use flammable liquids. Typically, this involves various combinations of purging with an inert gas, using explosion-proof electrical enclosures, or intrinsically safe wiring practices. All of these design considerations substantially increase the complexity and cost of utilizing alcohols with low flash points as cleaning agents. Further, alcohols which have traditionally been used as cleaning agents have relatively high vapor pressure, causing the release of relatively large amounts of vapors from cleaning operations.
Long-chain linear alcohols (seven or more carbon atoms) have higher flash points and lower vapor pressures than short-chain alcohols, resolving some of the limitations for using linear alcohols as cleaning agents. However, the long-chain linear alcohols are generally not miscible in water, and, therefore, require surfactants to allow mixing and rinsing with water. For example, U.S. Pat. No. 5,300,154 discusses, among other things, utilizing long-chain linear alcohols in combination with various terpenes, including limonene. These combinations do not appear to be water rinsable without the use of surfactants.
A recent advance in cleaning technologies has been the use of heterocyclic alcohols, including tetrahydrofurfuryl alcohol. Tetrahydrofurfuryl alcohol is used in biocide and pesticide formulations, paint stripping formulations, fabric dyes, and as a cleaning agent. Utilized as a cleaning agent, tetrahydrofurfuryl alcohol typically is blended with surfactants and water in a cleaning solution formulated to remove polar contaminants such as solder flux. Although this alcohol is completely miscible in water, the surface tension of tetrahydrofurfuryl alcohol is relatively high (37 dynes/cm) prompting the use of surfactants to lower the surface tension of the resulting solution and allow it to more readily wet the substrate being cleaned. Items cleaned with tetrahydrofurfuryl alcohol can be water rinsed and are generally considered organically and inorganically (ionically) clean, although the presence of surfactants presents contaminant problems similar to those discussed previously.
Ease of recycling of cleaning agents is important to reduce waste and because it often is more cost-effective to recycle cleaning agents rather than to continually replace used cleaning agents with new. In this regard, azeotropic or azeotrope-like compositions are desirable because they do not fractionate upon boiling. This behavior is desirable because azeotropic or azeotrope-like cleaning agents may be distilled for recycling. In comparison, preferential boiling of the components with lower-boiling points of non-azeotrope-like cleaning agents tends to result in distillate mixtures which have component fractions unlike the original cleaning agents. The changed composition may have less desirable or entirely different properties than the original. Therefore, azeotropic and azeotrope-like compositions enhance recycling.
Thus, there is a need for environmentally-friendly, easily recyclable, preferably azeotropic or azeotrope-like, low vapor pressure, high flash point cleaning agents which effectively remove both polar and non-polar contaminants without use of surfactants. The present invention addresses this need.